Antenna axis offset and intersection determination using GPS

Modern geodesy utilises advanced space techniques such as VLBI to further the study of crustal motion as well as tidal and rotational deformations of the Earth. One of the parameters in the model used to determine the baseline length between VLBI stations, is the antenna axis offset. This offset is the distance between the secondary axis and the normal projection of the secondary axis onto the primary axis. For a non-intersecting axes antenna mounting, this offset is usually several metres in length. The accuracy of the offset value directly influences the total accuracy of the VLBI results. This work describes how GPS is used to determine the offset and VLBI reference point. Several algorithms for calculating these parameters are investigated and evaluated. Methods are developed to minimise the size and influence of errors. An evaluation and comparison of the results to previous independent determinations of the offset, which use different techniques and instrumentation, clearly show the feasibility of using GPS. This method has the added advantage of allowing the VLBI, SLR and GPS reference frames to be co-located.